Slush-molding method for the manufacture of imitation leathers, in particular for the upholstery of cars

ABSTRACT

A slush-moulding method combined with cross-linking, including the filling of a tank with a suitable quantity of ground polymer blend and the coupling of said tank with a heated mold is provided. The closed system thus obtained is moved in order to transfer the powder onto the mold, so as to carry out on the latter a layer of completely or partially melted powder. Before and after a stage of stripping of the leather off the mold, said leather is cross-linked by means of radiations.

The present invention relates to a slush-molding method for themanufacture of imitation leathers, in particular for the upholstery ofcars.

As generally known, the slush-molding technology is broadly used for themanufacture of imitation leathers for the upholstery of cars, and inparticular for the covering of dashboards and door panels. Saidtechnology allows to obtain even highly three-dimensional imitationleathers easily and with low costs; moreover, slush molding gives theleather a leather design of excellent quality, and also allows to carryout even two-color leathers.

The slush-molding method is well known and reported in scientificliterature, for instance in the following recent articles:

Pabst H. G., Shaper S., Schmidt R., Terveen A. (AUDI—VW Group)—VDIConference—Mannheim 1988—“Development and large-scale use of dashboardsproduced by Slush Molding PVC”

Khue N. N., Kunper-Martz M., Dankmeier O., (EVC Group)—“Development ofoptimum powder blend for slush molding”—4^(th) International Conferenceon PVC 1990—Apr. 24^(th)-26^(th) 1990, Brighton U.K. pp. 31/1-31/14.

The following patents also contain descriptions related to slushmolding:

FR-A- 916.055, FR-A 1.131.153, FR-A 1.381.850, FR-A 1.560.675, U.S. Pat.No. B2,736,925, U.S. Pat. No. B3,039,146, GB-A-865.608, GB-A-1.025.493,GB-A-1.056.109, GB-A-1.337.962, DE-A-1.554.967, DE-A-3.417.727,IT-A-22197 A/80, EP-A-0 339 222, EP-A-0 476 742, and so on.

The aforesaid molding technology is based on a method including thefollowing stages:

a) an open-air tank is first filled with a suitable polymer powder in asufficient quantity and with grain sizes typically below 500 μm;

b) a mold, usually electroplated with nickel, is then heated to a giventemperature;

c) the tank and the mold are then coupled in a closed system withsuitable coupling means;

d) the system is moved so that the tank transfers the powder onto themold, thus obtaining a uniform layer of partially or completely meltedpowder which adheres to said mold;

e) the closed system is then opened after being brought to the initialconditions again; at this stage the possible excess polymer powderdeposits again into the tank and can thus be regenerated;

f) the mold can now be heated in order to complete the melting;

g) the mold is then cooled with suitable cooling means;

h) the formed leather is stripped off as a semi-finished product whichcan then be assembled with a support in order to obtain the finishedproduct in the form of dashboard, door panels, and so on, for theupholstery of cars.

The greatest success recently obtained with imitation leathers for cardashboards has been reached by slush molding of PVC powders, withoperations allowing to obtain even one- or multicolored leathers.

A problem which is still to be solved is how to obtain leathers by meansof slush molding of materials other than PVC (polyvinyl chloride) withlow costs, so as to avoid all the environmental and recycling problemarising from the use of a halogenated plastic material such as PVC.

PVC resins, suitably plasticized and having originally a sufficientlyhigh molecular weight, when worked, gel and reach a minimum viscosityvalue allowing it to spread onto the mold without any shear efforts(typically desirable characteristic if the slush-molding technique isused), whereas the other plastic materials cannot combine a highmolecular weight with such a low melt viscosity to give rise to the samephenomenon. As a matter of fact, high molecular weights, which wouldallow to obtain good performances as far as mechanical qualities andheat resistance are concerned, are also characterized by a very highmelt viscosity, thus preventing the distribution and the spreading ontoa mold without any shear effort.

On the other hand, the absence of shear efforts prevents the leatherobtained by slush molding from having unwanted frozen inner stresses.Obviously, the possible presence of inner stresses would first reducethe resistance and non-deformability according to temperature. Moreover,leather in such conditions would show an increase of shrinkage in thosecritical situations of ageing which are required by car makers.

The use of techniques other than slush molding for the manufacture ofleather result in the abovementioned disadvantages.

In particular, the kind of “hand” which can be obtained with theaforesaid slush-molding technique cannot be easily imitated with othertechniques, such as plate thermoforming. Moreover, the kind of leatherdesign, which is perfectly reproduced with slush molding and is notdeformed as in thermoforming, cannot practically be obtained with othertechniques. Within the same slush-molding technology various attemptshave also been made to replace PVC with other materials.

For instance, thermosetting liquid polyurethanes are now sprayed onto ametal mold. Said materials are first particularly expensive with respectto PVC and THEREFORE make the process more expensive than the use ofPVC. In addition, the products obtained by means of said materials haveshown several faults, such as the presence of inhomogeneous leathershims and the imperfect compacting of leather resulting in a decrementof the mechanical performances of the product.

Moreover, the corner portions and the possible undercuts on the leatherforming mold are typically concerned, during the deposit of liquidpolyurethane, by areas with an excess filling of material, which causesfurther imperfections on the product and scraps.

Powdered thermoplastic polyurethanes (TPU) have also been used, buttheir cost is again very high and they are not well accepted since theirrecycling is difficult and they cause toxic decomposition products.

On the basis of the existing art, briefly described above, and of thedisadvantages resulting from it,the preferable kinds of products in casePVC should be replaced, both for environmental and for economicalreasons, are those made of polyolefins, which have long been focused onby technicians.

The problems arising from the use of polyolefin materials are differentbecause of several reasons.

First of all, polyolefins show a low melting point (only homopolymerpolypropylene, or polypropylene copolymerized with low amounts ofcomonomers, would be suitable, but it is often stiff and does not show agood resistance to ageing, to solvents, to abrasion and to scratching);their molecular weights are low because of that, and therefore, thematerials offer a lower resistance to ageing.

Copolymerization further lowers their melting point, thus limiting theresistance to high temperatures which is required in specifications.

In case of thermoforming from calendered or extruded plates, theaforesaid disadvantages can be partially overcome, since the initialmolecular weights are high, with all the advantages resulting from thatas far as mechanical resistance, resistance to ageing and heatresistance, etc., are concerned.

To overcome the above problems are already know polyolefinscross-linking methods giving to the obtained semi-finished product ahigher heat resistance. For instance, the Japanese published patent No.JP 01275640 describes a copolymer of alpha-olefin with butene-1 andother comonomers with the addition of peroxides, in order to obtain anexpanded product which is resistant to high temperatures. This patentbriefly mentions the possibility to obtain cross-linking withradiations.

Another Japanese published patent, No. JP 63069837, pursues the same aimas the previous one with an expansible composition containing organicperoxides, said composition being extruded without expansion andexpanded after cross-linking, obtained with β-radiations (1 to 30 Mrad)with a cross-linking degree from 30 to 65% (gel fraction).

The Japanese patent JP 57197161 describes a laminated film ofpolyolefins or any kind of copolymers thereof (with vinyl acetate,octene, butene, etc.), said film having a high mechanical resistance, ahigh resistance to heat and a good weldability, cross-linked with dosesfrom 1 to 10 Mrad (1 Mrad=10 kGy=10⁴ J/kg) of X-rays, β-rays or γ-rays.

With the same aims as the previous patents, the patent JP 51092855claims the use of triallylisocyanurates and 1,2-polybutadiene added topolyethylene in order to help cross-linking with low doses ofradiations.

The Italian patent MI-A-000928 describes a half cross-linked film madeof an ethylene/vinyl carboxylate copolymer (for instance EVA), which canbe used as a surrogate of natural leather for coverings and othermanufactured items, characterized in that it is cross-linked with dosesof β-radiation from 30 to 70 kGy.

Said patent claims a half cross-linked film made of at least oneethylene/vinyl carboxylate copolymer, with 10-30% in weight of VCmonomer (vinyl carboxylate), formed with a blow extrusion technique orflat-head extrusion without cross-linking, and then cross-linked withradiations having said intensity. Moreover, the EVA copolymer, theformulation which can be used, its use as imitation leather and theitems manufactured from it are also claimed.

The systems of cross-linking with radiations which are used in knownsolutions have always been combined with plates or films, i.e. tosubstantially bidimensional manufactured items. Moreover, thoughimproving the mechanical, ageing, etc. characteristics, thecross-linking obtained with known systems does not always meet thespecification requirements of car industries. Other cross-linkingtechniques, mainly chemical, such as silanization, the use of peroxides,etc., show several problems, such as the need for environmentalconditions of strong humidity and high temperatures for long times(silanes), the need for the oxidation of the materials during theprocess, etc. (peroxides), and the problem concerning the preparation ofthe initial compound, which reduce their possibilities of use in thespecific field of slush molding.

The problem concerning the particular slush-molding process is thereforestill unsolved, i.e. how to obtain manufactured items with highperformances using polymers with a sufficiently low molecular weight,and therefore with suitable melt viscosity, said polymers preferablybeing ethylene polymers or copolymers mainly ethylene based.

The aforesaid general problem and the other problems which are typicallyrelated to the known technique are substantially overcome by theslush-molding process for the manufacture of imitation leather, inparticular for the upholstery of cars, as described in the accompanyingclaims.

In greater detail, the process according to the invention substantiallycombines two essential phenomena; the cross-linking with radiations asthe only means to increase the resistance of the manufactured article,in particular to high temperatures, and the radiation intensity, whichis higher than the known technique (also above 100 kGy); according tothe invention, moreover, the radiation, preferably a β-radiation, doesnot necessarily operate onto a flat film, but on slush-molded leatherswhich are already in the form of a finished part.

The need to use radiations with higher intensity results from the use ofmaterials with very low molecular weights. According to the invention,the recovery of the necessary mechanical characteristics and of thecharacteristics of resistance to high temperatures is obtained after theslush-molding process with an application of β-rays above the one whichis usually used on film material obtained by means of calendering orextrusion.

According to a preferred solution of the invention, the radiation can beused for the abovementioned purpose both when the leather still adheresto the mold after cooling, possibly on the manufacturing line, and afterpeeling said leather off the mold.

In the process according to the present invention it is possible to useall the polymers or copolymers which can be cross-linked by means ofradiations and which are suitable for slush-molding techniques thanks totheir characteristics of melt viscosity.

Said polymers are preferably olefin homo- and copolymers, and morepreferably ethylene polymers or copolymers with a high content ofethylene.

Further characteristics and advantages will be more evident from thedetailed description of some preferable non-limiting examples ofembodiment of a slush-molding process for the manufacture of imitationleathers, in particular for the upholstery of cars, according to thepresent invention.

The slush-molding process for the manufacture of imitation leathersaccording to the invention requires that a suitable quantity of apolymer blend made of olefin-based homo- and copolymers is firstprepared. Said blend is then reduced to a powder with reduced grainsize.

Obviously, in case the intention is to manufacture with the presentprocess a two-layer leather (visible compact layer plus inner expandedlayer) or a two-color leather, it will be necessary to use a first and asecond molecule, both provided with the necessary chemical, physicaland/or chromatic characteristics.

Generally, the preparation of a slush-molding powder with a particlesize almost always below 500 μm is carried out in this way:

First, with an extruder-granulator, a blend in grain form containing allthe necessary ingredients is prepared; the grains thus obtained areground, for instance in a cryogenic way (with liquid nitrogen) oralternatively with disc grinding (both at room temperature and at lowtemperature) in order to obtain the sieved powder, which is useddirectly for the slush-molding process.

It should be noted that, instead of grinding, it is possible to move onto micro-granulation directly during extrusion.

After the preparation of the polymer blend powder made of olefin-basedhomo- or copolymers, a sufficient quantity of said polymer powder isdeposited into a tank.

At this time or during the filling of the tank, a forming mold is heatedto a given temperature; it should be noted that the mold has the sameshape as the dashboard, the door inside panel or any other component ofthe car interior which should be covered with leather during themanufacturing stage.

It should be noted that the mold is preferably electroplated withnickel, even if it can obviously be carried out in other ways withoutleaving the idea described in the present invention.

The tank containing a suitable quantity of polymer powder and the abovedescribed mold are then reciprocally coupled with suitable couplingmeans so as to define a closed system.

The closed system is then moved from an initial condition to aturned-over condition in which said tank transfers the powder onto thesuitably heated mold by means of gravity, thus obtaining a uniform layerof completely or partially melted powder adhering to at least a portionof the shaping surface of said mold.

The closed system is then moved again and brought back to the initialconditions, so that the possible excess powder which has not adhered tothe mold deposits again into the tank and can thus be regenerated.

The closed system can now be opened and the mold can possibly bere-heated in order to complete the melting; this stage can be notessential if the powder material has been suitably melted in theprevious heating.

If a two-color manufactured item has to be produced, the first powdershould be deposited onto a limited portion of the shaping surface of themold, and after discharging the possible excess of first powder andcoupling the mold with an auxiliary tank, an additional layer of asecond powder, chromatically different from the first one, should bedeposited.

The second layer will cover the remaining portion of the shapingsurface, thus giving rise to a leather which, at least on the visibleside, will be two-colored.

If, alternatively, a two-layer leather has to be manufactured, a firstpowder defining a compact layer on the visible side should be deposited,followed by a second powder defining an expanded inner layer.

In any case, the following stages of the process include the moldcooling by suitable cooling means and the stripping-off of the formedleather as a semi-finished product which can be assembled, for instance,with a support so as to obtain the finished product in the form of adashboard, door panel, etc., for the upholstery of cars.

According to the invention, before stripping the leather off the moldor, alternatively, after said stripping-off, said leather iscross-linked with radiations, preferably including a β-radiation.

Obviously, if a two-layer manufactured item has to be carried out (bothlayers being compact, possibly with different colors, or a compact layerand a following inner expanded layer), the slush-molding process will beexecuted twice with a final cross-linking with β-rays which can involveboth layers.

In particular, the β-radiation having the necessary cross-linking effecthas proved to be the one whose intensity is between 20 and 300 kGy.

More precisely, the cross-linking effects which are necessary to ensuregood performances of the manufactured item have been obtained by meansof β-radiations with intensities above 70 kGy.

Always according to the invention, it has been surprisingly noted thatthe polymers which have proved ideal for the production of manufactureditems with the described process are made of blends, mainly polyolefinblends, with a melt viscosity value corresponding to a MFI (Melt FlowIndex) at 190° C. and under a load of 2,16 kg above 10 g/10 minutes.More precisely, said polymers are preferably olefin homo- andcopolymers, and more preferably ethylene polymer or copolymers with ahigh content of ethylene.

EXAMPLE 1

An ethylene-octene copolymer designated as “ENGAGE 8401”, produced byDouPont Dow Elastomers, bought in grains, was used.

Among additives, carbon black was added in percentages between 0,2% and2%, together with commercial antioxidants and UV absorbers in order toensure a better resistance of the material to light and heat.

In order to help the peeling of the leather, inner peeling agents can beadded, such as silicon peeling agents or waxes orlithium/calcium/magnesium stearates/laureates. The MFI of the copolymermeasured at 180° C. and under a load of 2,16 kg is 25 g/min.

The molecule is then reduced to a powder by grinding it until a graindiameter below 500 μm is reached.

By means of the abovementioned mixture a semifinished product wasobtained with the slush-molding technique previously described.

Said product then underwent cross-linking by means of β-rays having aradiation intensity of 80 kGy.

The cross-linking degree reached was 55%.

EXAMPLE 2

In a parallel a say carried out with the same materials and conditionsas the previous assay, changing only the radiation intensity, brought to120 kGy, the cross-linking degree was 66%.

The table below provides by way of example some data referring to theassays before cross-linking, after cross-linking and after ageing inoven at 130° C. for 240 h.

After cross-linking Cross-linking Breaking load Hardness Example degreeMPa ShA Untreated 0 8.5 78 Engage 8401 Engage 8401 55 11 76 Radiation:80 kGy 11 76 Engage 8401 66 12 75 Radiation: 120 kGy After cross-linkingat 130° C. 240 h Cross-linking Breaking load Hardness Example degree MPaShA Untreated 0 Loss of structural Engage 8401 characteristics Engage8401 55 10 76 Radiation: 80 kGy Engage 8401 66 9 79 Radiation: 120 kGy

These data show that the semifinished product, after undergoing thecross-linking process, shows an increment of the breaking load. Thisvalue moves from 8,5 MPa of the non cross-linked material to 11 MPa, ifthe cross-linking degree is 55%. If, on the other hand, thecross-linking degree is 66%, this value reaches 12 MPa.

Even after the ageing process at 130° C., the values of breaking loadare 10 MPa (cross-linking 55%) and 9 MPa (cross-linking 66%).

Hardness keeps within ranges of small variation, with values between 75and 79 ShA.

EXAMPLE 3

The starting material is now a blend of an ethylene/vinyl acetatecopolymer (EVA), in which the weight percentage of vinyl acetate is 28%,with low-density polyethylene (PE) in a 80:20 ratio.

The MFI (melt flow index) of the copolymer measured at 190° C. and undera load of 2,16 kg is 20 g/10 min.

The blend before grinding is further added with pigments (carbon black),peeling agents, antioxidants, UV absorbers and cross-linkingaccelerators.

The blend is then reduced to powder.

After manufacturing the semifinished product with the slush-moldingtechnique using said mixture, said semi-finished product undergoescross-linking by means of β-radiation.

The assays with β-radiation having an intensity of 80 kGy allowed toobtain a cross-linking degree of 65%.

On the other hand, the assays with β-radiation having an intensity of120 kGy showed a cross-linking degree of 75%.

The table below provides by way of example some data referring to theassays with the blend of ethylene/vinyl acetate copolymer andlow-density polyethylene.

After cross-linking Cross-linking Breaking load Hardness Example degreeMPa ShA Untreated 0 5 69 EVA + PE EVA + PE 65 7 72 Radiation: 80 kGyEVA + PE 75 9 75 Radiation: 120 kGy After cross-linking at 130° C. 240 hCross-linking Breaking load Hardness Example degree MPa ShA Untreated 0Loss of structural EVA + PE characteristics EVA + PE 65 6 74 Radiation:80 kGy EVA + PE 75 8 77 Radiation: 120 kGy

What is claimed is:
 1. A slush-moulding method combined with thecross-linking including the following steps: preparing of a suitablequantity of at least a first polymer blend; grinding or microgranulating said first blend in order to obtain a powder; charging intoat least a tank of a sufficient quantity of polymer powder; heating amould to a given temperature; coupling said tank with said mould withsuitable coupling means; moving the reciprocally coupled tank and mouldfrom an initial condition to a turned-over condition in which the tanktransfer at least part of the powder into the mould, thus obtaining alayer of completely or partially melted powder adhering to at least aportion of the shaping surface of the mould; further possible heating inorder to complete the melting; cooling the mould so as to obtain asemi-finished product as a film; stripping-off the semi-finished productas a film, wherein before or after the stripping-off step, the semifinished product as a film is cross-linked by means of a radiationhaving an intensity between 70 and 300 kGv in order to obtain a coveringletter.
 2. A slush-moulding method combined with the cross-linkingincluding the following steps: preparing a suitable quantity of at leasta first polymer blend; grinding or micro granulating said first blend inorder to obtain a powder; charging into at least a tank of sufficientquantity of polymer powder; heating a mould to be given temperature;coupling said tank with said mould with suitable coupling means; movingthe reciprocally coupled tank and mould from an initial condition to aturned-over condition in which the tank transfer at least part of thepowder into the mould thus obtaining a layer of completely or partiallymelted powder adhering to at least a portion of the shaping surface ofthe mould; further possible heating in order to complete the melting;cooling the mould so as to obtain a semi-finished product as a film;stripping off the semi finished product as a film, wherein, before orafter the stripping-off step, the semi-finished product as a film iscross-linked by means of a β radiation in order to obtain a coveringleather.
 3. A method according to claim 1, wherein said polymer blendcontains olefin-based homo-or copolymers.
 4. A method according to claim1, wherein the blend is mainly polyolefin-based, with a value of meltviscosity corresponding to a MFI (Melt Flow Index) at 190° C and under aload of 2,16 Kg above 10 g/10 minutes.
 5. A method according to claim 4,characterized in that said polymers are preferably olefin homo- orco-polymers, and more preferably ethylene polymers or co-polymer with ahigh content of ethylene.
 6. A method according to claim 1, whereincross-linking takes place before stripping the leather off the mould. 7.A method according to claim 1, characterized in that wherein the crosslinking takes place after the stripping of the semi-finished products asa film off the mould.
 8. A method according to claim 4, wherein theblend, mainly polyolefin-based, contains an ethylene/vinyl acetatecopolymer.
 9. A method according to claim 4, wherein the blend, mainlypolyolefin-based, contains an ethylene/octene copolymer.
 10. A methodaccording to claim 1, wherein after said moving step before saidstripping-off, the reciprocally coupled mould and the tank are broughtback to the initial condition so as to determine the discharge of theexcess powder from the mould towards the tank.
 11. A method according toclaim 1, wherein the preparation, grinding or micro granulation,deposition, heating, coupling, moving and cooling are carried out oneafter the other.
 12. A method to claim 1, wherein before the coolingstep the following steps are carried out: disengagement of the mouldfrom said tank; coupling of the mould with an auxiliary tank housing agiven quantity of a powder of a second polymer blend; moving of theauxiliary tank and of the mould reciprocally coupled from an initialcondition to a turned-over condition in which the auxiliary tanktransfers at least part of the second powder into the mould, thusobtaining a layer of completely or partially melted powder into themould, thus obtaining a layer of completely or partially melted powderwhich at least partially overlaps the previously deposited layer.
 13. Amethod according to claim 12, wherein the powder of the second polymerblend shows a higher expansibility than the polymer powder previouslydeposited.
 14. A method according to claim 12, wherein the layer offirst powder adhering to the mold covers a portion of the shapingsurface of said mold, said layer being obtained with the second powdercovering at least the remaining portion of the shaping surface of themold.
 15. A method according to claim 14, wherein the layer obtainedwith the second powder is chromatically different from the layerobtained with the first powder, thus obtaining leathers with differentcolors.
 16. A method according to claim 1, wherein said mould shows athree-dimensional shaping surface for a leather also having athree-dimensional development.